K. K. SOBIYI et al.: THE INFLUENCE OF SCANNING SPEED ON THE LASER METAL DEPOSITION ...
345–351
THE INFLUENCE OF SCANNING SPEED ON THE LASER METAL
DEPOSITION OF Ti/TiC POWDERS
VPLIV HITROSTI SKENIRANJA NA LASERSKO DEPOZICIJO
Ti/TiC PRAHU NA KOVINO
Kehinde Sobiyi, Esther Akinlabi, Stephen Akinlabi
University of Johannesburg, Department of Mechanical Engineering Science, Auckland Park Campus, 2006 Johannesburg, South Africa
kk_sob@yahoo.com
Prejem rokopisa – received: 2016-04-07; sprejem za objavo – accepted for publication: 2016-05-10
doi:10.17222/mit.2016.062
The paper describes experimental work performed on the laser metal deposition (LMD) of titanium carbide powders on a pure
titanium substrate. The understanding the effect of LMD processing parameters is vital in controlling the properties of the final
product fabricated from the LMD process. The objective of the study is to characterize the influence of the laser scanning speed
of the metal deposition of titanium and titanium carbide powders on a pure titanium substrate. Microstructural results showed
that the substrate is characterized by two-phase morphology; alpha and beta phases. The deposit zone microstructures showed
that the grains are of continuous columnar in nature. The heat-affected zone region grain areas appear to decrease with
increasing in scanning speed for different samples at different scanning speeds. The height of samples was observed to decrease
with an increase in the scanning speed. The microhardness results showed that the hardness of the deposits is greater than the
hardness of the substrate. Wear-resistance performance results showed that the coefficient of friction of the substrate is greater
than the coefficient of friction of the deposit samples. Similarly, the wear volume loss of material of the substrate is higher than
the deposits. The deposit contains titanium carbide and, as such, this powder has improved the wear resistance performance of
the substrate.
Keywords: titanium, lasers, metal deposition, scanning speed
^lanek opisuje eksperimentalno delo pri laserskem nana{anju (angl. LMD) prahu titanovega karbida na podlago iz ~istega
titana. Upo{tevanje u~inkov LMD procesnih parametrov je klju~no za kontrolo lastnosti kon~nega proizvoda, izdelanega z LMD
postopkom. Namen {tudije je dolo~iti vpliv hitrosti skeniranja laserja na nana{anje prahu kovinskega titana in titanovega karbida
na podlago iz ~istega titana. Rezultati mikrostrukturne karakterizacije so pokazali, da je za podlago zna~ilna dvofazna
morfologija; alfa- in beta faza. Mikrostruktura nane{ene plasti je pokazala, da so zrna obi~ajno stebraste strukture. Zrna v
podro~ju toplotno vplivane cone se zmanj{ujejo z nara{~anjem hitrosti skeniranja, pri razli~nih vzorcih in razli~nih hitrostih
skeniranja. Vi{ina vzorcev pri razli~nih hitrostih skeniranja, se je zmanj{evala z nara{~anjem hitrosti skeniranja. Rezultati
mikrotrdote so pokazali, da je trdota nanosa ve~ja od trdote podlage. Obna{anje pri obrabi je pokazalo, da je koeficient trenja
podlage ve~ji kot pa koeficient trenja nanosa. Izguba materiala zaradi obrabe je ve~ja pri podlagi kot pa pri nanosu. Nanos
vsebuje titanov karbid, zato je ta prah pove~al obrabno odpornost podlage.
Klju~ne besede: titan, laserji, nana{anje kovine, hitrost skeniranja
1 INTRODUCTION
Titanium and its alloys are becoming widely used
engineering materials, most especially in the aerospace,
biomedical and chemical industries, due to its impressive
mechanical properties such as, high-strength-to-weight
ratio, high-temperature strength and excellent corrosion
resistance.1,2 However, the use of titanium in severe wear
applications is limited due to its poor tribological pro-
perties.3,4 Titanium alloys are very expensive to replace
when damaged during their service life as a result of
their high cost.
The Laser Metal Deposition (LMD) technique is
efficient, cost effective, and environmentally friendly
used for repairing or modifying components that were
worn during service by improving its surface properties.3
The technique can be used for modifying the surfaces of
titanium alloys using powder metals such as, ceramics,
metals and alloys, composites and intermetallics, for
improving its wear resistance, medical biocompatibility,
corrosion and oxidation resistance.5
Several processing parameters such as laser power,
scanning speed, powder rate, or gas flow rate are im-
portant during LMD.6,7 However, the efficiency of the
LMD process is affected by the processing parameters
which, in turn, affect the physical, microstructural, and
microhardness profiling properties. Understanding the
effects of these processing parameters during the LMD
process, assist on controlling the resulting material pro-
perties. Although several studies have been performed to
understand the effect of laser process parameters on the
LMD process, only few attempts have been made to im-
prove the wear properties of commercially pure titanium.
This study presents the effect of scanning speed on
the microstructure and the wear of the resulting deposit
using the LMD technique.
Materiali in tehnologije / Materials and technology 51 (2017) 2, 345–351 345
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 621.762:621.6.04:669.295 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 51(2)345(2017)
2 EXPERIMENTAL PART
LMD of titanium (average particle size of 60 μm) and
titanium carbide (average particle size of 200 μm)
powders on the surface of 99.6 % pure titanium substrate
(75 mm × 75 mm) was achieved using a Kuka robot, 4.4
KW Nd-YAG laser deposition machine. The experimen-
tal setup consisted of the following subsystems operating
in a control system: the laser beam attached to the robot
arm, dual powder feeding system hoppers (for the
titanium and titanium carbide powders), and the control
unit. The deposition was performed using a laser focal
length of 195 mm and a beam diameter of 2 mm. Figure 1
is a schematic diagram of the Kuka robot system used in
the LMD process.
At the beginning of the experiment, a pre-test inspec-
tion is performed to eliminate factors that might affect
the results. The substrate was sand blasted and then
rinsed in acetone to clean the surface before the depo-
sition process. After sand blasting, the powders were
deposited on the surface of the substrate.
Six samples were prepared, each at a different laser
scanning speed. The scanning speeds were varied from
0.3 m/min and increased with an increment of 0.2 m/min
until 1.3 m/min. The experimental matrix is presented in
Table 1. The laser power and powder volume flow rate
for the powders were kept constant at 1200 W and
0.3 min–1, respectively.
The cladding powders were shielded by argon gas at
a flow rate of 2 L/min while depositing on the surface of
the substrate. The oxygen level was kept in the 10 min–1
level using a glove box. The deposition process was
achieved by feeding the powders onto the melting pool
on the surface of the substrate.
Table 1: Experimental matrix
Tabela 1: Matrika preizkusov
Samples Scanning speed(m/min)
Ti powder flow
rate (min–1)
TiC powder
flow rate (min–1)
A 0.3 1.7 0.3
B 0.5 1.7 0.3
C 0.7 1.7 0.3
D 0.9 1.7 0.3
E 1.1 1.7 0.3
F 1.3 1.7 0.3
Microscopic examination was performed on the laser
deposit, to examine the defects that might affect the pro-
perties of the materials. Characterization was performed
in accordance with the ASTM E3-11 standards for pre-
paring the samples.
Small pieces of samples for each scanning speed
were cut from the substrate, cleaned, mounted using
polyfast resin. The substrate was grinded to achieve a
smooth surface, using silicon carbide paper (320 grit),
MD-largo and DiaPro. Polishing was done using
MD-Chem cloth with OP-S mixture as the suspension.
Each sample was etched for 25 s using Nital solution.
The microstructure and the surface topography of the
substrate were observed using an optical microscope
(Olympus BX51M). Microhardness was performed using
a Vickers testing machine (EMCOTEST®), and a test
load of 500 g/f was used. Ten indentations were applied
per sample and the average diagonal values of the sam-
ples were recorded. The micrograph of the wear scars
was observed under the TESCAN scanning electron
K. K. SOBIYI et al.: THE INFLUENCE OF SCANNING SPEED ON THE LASER METAL DEPOSITION ...
346 Materiali in tehnologije / Materials and technology 51 (2017) 2, 345–351
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: Kuka robot system used for laser deposition process
Slika 1: Kuka robotski sistem, uporabljen pri postopku nana{anja z laserjem
microscope (SESCAN) equipped with an Oxford Instru-
ments energy-dispersive X-ray spectrometer (EDS).
A dry sliding wear test was performed on the depo-
sited surfaces to investigate the wear resistance of the
substrate at different laser scanning speeds, according to
ASTM G133-05 standard using a pin-on-disk tribometer
(by Cert). The sample was preloaded with a tungsten
carbide ball of diameter 10 mm at 25 N force for the 10
s, and then loaded with the same force 25 N for 1000 m.
The coefficient of friction, lateral force, horizontal force
and carriage position were plotted against time for 1000
seconds. The wear volume loss of material on the
substrate is calculated from Equation 1:
V V V L r
W
r
W
r
W
total A B= + =
⎛
⎝
⎜ ⎞
⎠
⎟ − −
⎛
⎝
⎜
⎞
⎠
⎟
⎧
⎨ −2 1 2
2 1 2
2 2 4
sin
/
⎩
⎫
⎬
⎭
+
+ − −
⎛
⎝
⎜
⎞
⎠
⎟ − −
⎛
⎝
⎜
⎞
⎠
⎟
⎧π
3
2 2
4 4 4
3 2 2
2 1 2 2
2
2 1 2
r r r
W W
r
W
/ /
⎨
⎩
⎫
⎬
⎭
(1)
3 RESULTS AND DISCUSSION
3.1 Effect of speed on laser metal deposition
Figure 2 shows the physical appearance of the
deposits on the substrate after LMD of the titanium
carbide powders. The thicknesses of the layers varies
with scanning speeds. At a low scanning speed the
thickness appears to be bigger and wider than the
thickness at a high scanning speed. Observed the texture
of the surface at lower scanning speed appears more
rough compared to the surface texture at higher scanning
speeds. This is probably due to more powders were
melted on the surface of the substrate, thus enhancing the
surface texture. The microstructure of the laser deposit
was observed to be relatively homogeneous. The depo-
sits were generally free from cracks but pores were
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Figure 4: Morphology of the samples with deposit height
Slika 4: Morfologija vzorcev z vi{ino nanosa
Figure 3: a) Morphology of sample A, b) enlarged deposit zone and
diffusion zone, c) enlarged heat-affected zone
Slika 3: a) Morfologija vzorca A, b) pove~ano podro~je nana{anja in
podro~je difuzije ter c) pove~ano toplotno vplivano podro~je
Figure 2: Physical appearance of the deposits
Slika 2: Fizi~ni izgled nanosov
observed (Figure 4), caused by gas produced from the in
situ, entrapped within the molten pool during the depo-
sition process.4,8
Figure 3a shows the morphology of sample A and
Figure 3b the enlarged view of the deposit zone and
diffusion zone, at scanning speed of 0.3 m/min. The
deposit zone, diffusion zone and heat-affected zone
(HAZ) were observed in Figure 3. The deposit zone is
the layer of powder deposited on the substrate. The
diffusion zone is the mixture of the deposition powder
and the substrate material resulting after the laser metal
deposition process. The HAZ is the portion on the
surface of the substrate altered by heat during the
deposition process. The deposit zone and diffusion zone
appear to have a similar grains structure characterized by
the grains of the beta phase. The deposit zone and the
diffusion zone are all characterized by the grains of the
beta phase. From the microstructure, it can be noticed
that the deposit zone grains are of a continuous columnar
nature, confirming the beta phase. As the laser scanning
speed increases the alpha phase is noticed to be
developing.
With the other samples, the deposit zone region is
noticed to be decreasing with an increase in the scanning
speed. This is a result of heat received by the region
during deposition, which cause microstructural changes
in that region. As the scanning speed increases the time it
takes for the laser to complete the deposit decreases. It
can also be observed that as the scanning speed increases
the grains appear to be decreasing and more regions of
alpha phase are appearing on the microstructures of the
samples.
The grains on the heat-affected region appear to be
characterized by beta phase, similar to that observed by
R. M. Mahamood et al.9 Generally, at a low laser scann-
ing speed, there is a good bonding between the substrate
material and the deposit powder.9 The material of sample
A is fully dense compared to material of the sample
scanned at a high laser scanning speed. The optical
micrograph of the other five samples is shown in Figures
4b to 4f). Comparing Sample A and sample F in Figure
4f, it can be noticed that areas of HAZ are different. For
sample F the area is very small and for sample A the area
is large.
The area of the HAZ appears to decrease as the
scanning speed increases. The HAZ region of sample B
is characterized by a microstructure with grains less that
the grains of sample A. As the scanning speed increases
the melt pool gets bigger and the solidification becomes
slower, causing the materials of the substrate to melt
deeper, causing the substrate to mix more of its material
with the deposit material, thus, the deposit height de-
creases.10
The HAZ area of sample F where the scanning speed
is 1.3 m/min is characterized with fewer refined grains
formed in that area compared to the HAZ of sample A
where the scanning speed is 0.3m/min. This is caused by
heat received by the area, which cause microstructural
changes in that area.
The deposit zone of sample A was measured to be
129.88 μm. The heights of the other samples (B–F) were
105.4 μm, 93.96 μm, 59.16 μm, 55.08 μm and 59.20 μm
respectively. Comparing measured heights of all samples
and height of sample A it can be seen that the height
decreases with increase in scanning speed.
K. K. SOBIYI et al.: THE INFLUENCE OF SCANNING SPEED ON THE LASER METAL DEPOSITION ...
348 Materiali in tehnologije / Materials and technology 51 (2017) 2, 345–351
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 7: Graphical representation of coefficient of friction (COF)
with time
Slika 7: Grafi~na predstavitev koeficienta trenja v odvisnosti od ~asa
Figure 5: Microhardness profile of sample A at a scanning speed of
0.3 m/min
Slika 5: Profil mikrotrdote vzorca A pri hitrosti skeniranja 0,3 m/min
Figure 6: Graphical representation of average hardness for all the
samples
Slika 6: Grafi~na predstavitev povpre~ne trdote za vse vzorce
Figure 5 shows a graphical representation of the
microhardness profile measured across the section of the
sample A from top to bottom. The first three hardness
measurements were taken on the deposit zone region, the
next three measurements were taken on the HAZ region
and the last three measurements were taken on the sub-
strate material region. From the graphical representation
of the microhardness profile results, it can be observed
that the hardness increases initially and then later
decreases towards the substrate. The higher hardness
experienced at the transition between the deposit and
HAZ zone can be attributed to possibly better
crystallization of the grains in this zone. Similar was
observed with the other five samples. This shows that the
addition of titanium carbide powder improves the
mechanical properties of the substrate, and the heat
generated by the laser affects the mechanical properties
of the substrate.
Figure 6 shows the experimental average microhard-
ness for all six samples. From the graphical represen-
tation of the results it can be noticed that the average
hardness increased with an increase in the scanning
speed. At a higher scanning speed there is rapid solidifi-
cation of the deposit, which reduces the time taken for
the melt pool to solidify, thus promoting higher hardness.
3.2 Wear resistance behaviour
In Figure 7, the experimental coefficient of friction
for sample A and the substrate are shown for the dry
sliding wear resistance tests. From the graphical repre-
sentation of the results it can be noticed that the
coefficient of friction of the substrate is higher than the
coefficient of friction. Similar result was obtained for all
the other samples investigated.
The titanium wear resistance is found to be very poor
compared to the deposit layer as a result of its high
coefficient of friction and higher chemical affinity with
the tungsten carbide ball used for the wear test thus it is
expected for the substrate material to have a high wear
volume loss of material.10,11 The coefficient of friction of
the deposits of all other samples is lower than the one of
the substrate resulting from the melted TiC particles on
the surface. Titanium carbide powder was used as a
reinforcement in this study.
In Figure 8, the average coefficient of friction at in-
creasing scanning speed corresponding to the individual
samples is shown for the dry sliding wear performance
testing. The graphical representation of the results shows
that the coefficient of friction increases with an increase
in the scanning speed; as a result, sample A has a smaller
K. K. SOBIYI et al.: THE INFLUENCE OF SCANNING SPEED ON THE LASER METAL DEPOSITION ...
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Figure 10: SEM morphology of sample A wear scar: a) low magni-
fication, b) high magnification
Slika 10: SEM-morfologija podro~ja obrabe vzorca A: a) pri majhni
pove~avi, b) pri ve~ji pove~avi
Figure 8: Graphical representation of average coefficient of friction
Slika 8: Grafi~na predstavitev povpre~ja koeficienta trenja v odvisno-
sti od ~asa
Figure 9: Graphical representation of wear volume loss
Slika 9: Grafi~na predstavitev volumske izgube pri obrabi
coefficient compared to sample F. Consequently, the
substrate is observed to have the highest coefficient of
friction.
Figure 9 shows the experimental wear volume loss at
different scanning speeds. The figure shows a higher
volume of material removal from the surface of the
substrate compared to the volume loss of material of the
samples at different scanning speeds. The titanium wear
resistance is very poor, so it is expected that the substrate
material will have a high wear volume loss of material.
On the deposit the wear volume loss will be at a mini-
mum level as the deposit layers contain titanium carbide
powder melted on the surface of the substrate. The pre-
sence of titanium carbide reinforced composite layer
improved the wear resistance of the substrate.
Figure 10 shows the SEM micrograph of sample A at
low and high magnifications. From the figure, a large
scar was observed, with the appearance of large width
and long length, which is an indication of wear on the
surface of the sample. Un-melted particles of titanium
carbide powder were also observed on the surface of the
substrate, as shown in Figure 10a.
The morphology of wear scars for all other five sam-
ples are at higher magnification is shown in Figure 11.
Surface defects such as pits and wear debris were
observed along the wear tracks; an indicating abrasive
and adhesive wear mechanism which is coupled with
severe plastic deformation around the edges of the wear
scars.
4 CONCLUSION
The study investigated the characterization of the
influence of the laser scanning speed of LMD for tita-
nium and titanium carbide powders on a titanium alloy
substrate.
The results obtained through the characterization of
the influence of the laser scanning speed of the laser
deposition for titanium and titanium carbide powders has
shown that the laser process parameters have an effect on
the material properties of components subjected to the
LMD process. The microstructure of each sample of
different laser deposition scanning speeds showed that
the HAZ region increased with an increase in the scann-
ing speed and the heights of the deposits were observed
to be different for different laser scanning speeds of the
powder particles.
The microhardness results showed that different
zones on the substrate have different hardness properties.
The hardness decreases from the deposit zone, HAZ and
from the HAZ to the substrate. The microhardness was
found to increase with an increase in the scanning speed
for all the samples investigated. This was a result of the
time taken by the laser to interact with the substrate
material. These results showed that at a lower scanning
speed the laser interacts with the material for a long
period of time, which takes longer to solidify the
deposits, which in turn affects the results of hardness.
This shows that at a lower laser scanning speed the
degree of mixing for the substrate material with the
deposited powder is proper.
The results of the wear resistance performance
behavior of the samples at different scanning speeds and
K. K. SOBIYI et al.: THE INFLUENCE OF SCANNING SPEED ON THE LASER METAL DEPOSITION ...
350 Materiali in tehnologije / Materials and technology 51 (2017) 2, 345–351
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 11: SEM images of wear tracks at 2000×: a) sample A at 0.3 m/min, b) sample B at 0.5 m/min, c) sample C at 0.7 m/min, d) sample D at
0.9 m/min, e) sample E at 1.1 m/min, f) sample F at 1.3 m/min
Slika 11: SEM-posnetki sledov obrabe pri 2000× pove~avi: a) vzorec A pri 0,3 m/min, b) vzorec B pri 0,5 m/min, c) vzorec C pri 0,7 m/min, d)
vzorec D pri 0,9 m/min, e) vzorec E pri 1,1 m/min in f) vzorec F pri 1,3 m/min
the substrate indicates that the wear volume of material
removed from the surface of the substrate is higher than
the wear volume of the material removed from the depo-
sit surface. The coefficient of friction of the substrate
was observed to be higher than the coefficient of friction
of the samples at different scanning speeds. The depo-
sition of titanium carbide showed that the volume loss of
material from the substrate could be reduced. Titanium
carbide improved the wear resistance of the component.
The SEM analysis showed that at a low laser scanning
speed some powder particles are completely melted on
the surface of the substrate.
Acknowledgments
The authors would like to thank NLC Council for
Science and Industrial Research for the use of their Nd:
YAG laser deposition machine and Tshwane University
of Technology for their wear resistance testing apparatus.
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